Preserve BOM in web editor (#28935) (#28959)

Backport #28935 by @silverwind

The `ToUTF8*` functions were stripping BOM, while BOM is actually valid
in UTF8, so the stripping must be optional depending on use case. This
does:

- Add a options struct to all `ToUTF8*` functions, that by default will
strip BOM to preserve existing behaviour
- Remove `ToUTF8` function, it was dead code
- Rename `ToUTF8WithErr` to `ToUTF8`
- Preserve BOM in Monaco Editor
- Remove a unnecessary newline in the textarea value. Browsers did
ignore it, it seems but it's better not to rely on this behaviour.

Fixes: https://github.com/go-gitea/gitea/issues/28743
Related: https://github.com/go-gitea/gitea/issues/6716 which seems to
have once introduced a mechanism that strips and re-adds the BOM, but
from what I can tell, this mechanism was removed at some point after
that PR.

Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit b8e6cffd31)

web_src/js/features/codeeditor.js

@@ -114,7 +114,7 @@ export async function createMonaco(textarea, filename, editorOpts) {
 
   const model = editor.getModel();
   model.onDidChangeContent(() => {
-    textarea.value = editor.getValue();
+    textarea.value = editor.getValue({preserveBOM: true});
     textarea.dispatchEvent(new Event('change')); // seems to be needed for jquery-are-you-sure
   });